1. Field of the Invention
The present invention relates to a discharge-lamp lighting apparatus that lights a discharge lamp.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2005-507553 discloses a discharge-lamp lighting apparatus shown in FIG. 4. The discharge-lamp lighting apparatus lights a discharge lamp by applying an alternating-current (AC) voltage and uses a full-bridge inverter circuit. In the discharge-lamp lighting apparatus in FIG. 4, the AC voltage is applied to an LC series circuit and the discharge lamp is connected to both ends of a capacitor in the LC series circuit. Upon startup of the discharge lamp, an AC voltage having a high frequency near the resonant frequency of the LC series circuit is applied to the LC series circuit to supply a high voltage necessary to start up the discharge lamp. As shown in FIG. 5, the switching frequency in the inverter circuit is swept near the resonant frequency of the LC series circuit upon startup of the discharge lamp and is set to less than the resonant frequency for a certain time period after the sweeping. After the operation at startup of the discharge lamp is repeated once or a few times, the switching frequency moves to a frequency considerably less than the resonant frequency of the LC series circuit and the discharge lamp is lighted with a lower voltage.
The switching frequency in the inverter circuit is set to less than the resonant frequency for a certain time period after the sweeping in order to warm up the discharge lamp. This is done to prevent the discharge lamp from being turned off when a higher frequency at the startup moves to a lower frequency at the lighting.
However, since the switching frequency during the application of the AC voltage for a certain time period after the sweeping is set to less than the resonant frequency of the LC series circuit, destruction of the lamp or other problems may occur if the switching frequency is matched with an acoustic resonance frequency band specific to the lamp.